This invention relates to apparatus facilitating the processing of acoustic well-logging signals and, more particularly, to novel and highly-effective apparatus allowing the automatic correction of errors due to cycle skipping in the measurement of the transit time of the acoustic measurement waves used.
Acoustic well-logging methods and apparatus are usually designed to measure the transit time of an acoustic wave between a first and a second point.
French Pat. No. 1,349,989 discloses such a method employing two receivers placed a certain distance from each other, between two transmitters. With such apparatus, one measures the transit time between the two receivers for waves transmitted respectively by the first and by the second transmitter. By taking the average (i.e., the half-sum) of the transit time thus obtained, an accurate measurement is obtained independent, in particular, of the errors introduced by the tilting of the probe in the borehole.
There are various methods for measuring the transit time of an acoustic wave between a transmitter and a receiver. The method disclosed in the patent mentioned above involves triggering a timing-pulse counter the instant a given half-wave of a synchronous pulse representative of the operating point of the transmitter is applied to the counter's starting circuit and of stopping the counter when the acoustic measurement waves are received and a corresponding half-wave in the reception signal generated by the receiver is applied to the counter's stopping circuit. The standard shaping of the synchronous transmission pulse is easy, and, consequently, the starting of the counter takes place under favorable conditions. This is not true of the reception signal. Because of the presence of noise superimposed on the reception signal generated by the receiver, a detection threshold is set for the reception signal so that the counter is not likely to be stopped accidentally by the noise signals. Such noise signals are of two kinds: those picked up by the receiver and those picked up in the cable (crosstalk). The noise signals generated by the receiver are intermittent and can reach a very high amplitude. Such is the case when the probe strikes against the wall of the borehole and, to a lesser extent (longer duration and lower amplitude), when the probe rubs against a borehole wall which has an uneven surface or cavities. The crosstalk noise picked up in the cable is constantly present, and its amplitude is relatively low compared with that of the noise produced by probe impact against the wall of the borehole.
In practice, when use is made of a well-logging apparatus without any noise correction device, the operator verifies from time to time, on an oscilloscope, the quality of the reception signal received at the surface, and, when he notices that, within the region being surveyed, the average noise superimposed on the reception signal increases or decreases significantly, he modifies accordingly the threshold at which the useful half-wave of the reception signal is detected. Under these conditions, the instant the reception signal exceeds the threshold thus set, a stopping pulse is applied to the counter. Consequently, it is essential that the operator monitor the conditions under which the measurement is carried out. Such monitoring is tedious, and, in spite of the vigilance of the operator, it often happens that the threshold set for the detection of the measurement signals corresponds only belatedly to the instantaneous noise conditions at probe level. This can result in measurement errors.
Alternatively, the signal can in effect be monitored automatically. In this case, the amplitude of the detection threshold is automatically modified as a function of the instantaneous noise amplitude just before the transmission of the acoustic pulse whose transit time is measured. Such a method is described in a U.S. patent application of Bernard Vivet et al, Ser. No. 325,168 now abandoned in favor of continuation application Ser. No. 528,693, filed Dec. 2, 1974 and a U.S. patent application of Jean Hubert Guignard, Ser. No. 325,121, for "Acoustic Well Logging With Threshold Adjustment", both filed Jan. 19, 1973. Said application Ser. No. 325,121 is now abandoned in favor of continuation application Ser. No. 528,694, filed Dec. 2, 1974, in turn abandoned in favor of continuation application Ser. No. 678,643, filed Apr. 20, 1976.
These means, particularly the automatic means, for adapting the detection threshold to the amplitude of the noise prevailing at the same moment, produce a substantial improvement in the accuracy of the measurement. However, a further correction is sometimes required. Such is the case when the threshold is too low and leads to the stopping of the transit time counter on a noise signal appearing during the transmission-reception interval. This occurs for example when, with the threshold adjusted rather low, the probe strikes the borehole wall just between the acoustic wave transmission and reception instants. Statistically, such a case is relatively rare. On the other hand, a relatively frequent case is the one in which the signal/noise ratio (for the signal half-wave of interest) at the level of the receiver drops below unity. This may be the case when the formations traversed by the borehole have a high attenuation for acoustic waves and, in addition, a rough wall or more-or-less deep cavities. In this case, the rubbing of the probe or of its holding arms on the borehole wall generates, for several successive measurement signals, noise having an amplitude considerably greater than the amplitude of the half-wave of the reception signal used for measuring the transit time. Since the detection threshold is set as a function of the noise existing at that instant, the detection of the reception signal will then take place on the half-wave of the same polarity which follows--and which has an amplitude two to three times higher--, so that the measured transit time is increased by the duration of the average period of the transmitted acoustic waves. This well-known phenomenon is called cycle skipping and appears from time to time whatever the speed and the accuracy of the detection threshold adjustment as a function of noise. This cycle skipping error adversely affects the quality of the logging. It is of course possible for a skilled analyst to notice such an error when studying the recording, since it will then exhibit sudden transit-time variations of short duration. In the case where a differential measurement of transit time between two receivers is made, cycle skipping can lead to either an increase or a decrease in the measured transit time, depending on whether the error affects the remote receiver or the receiver near the transmitter.
When automatic regulation of the average amplitude of the useful half-wave is used, by controlling the gain of the downhole amplifier, as disclosed in the patent applications mentioned above, cycle skipping can lead to the locking of the entire measurement system on an incorrect half-wave of the reception signal. In this case, the gain of the downhole amplifier is decreased in accordance with the ratio of the amplitude of the second half-wave of chosen polarity (generally negative) of the reception signal to the amplitude of the first half-wave of the same polarity. The result is then a series of erroneous measurements which are practically impossible to detect.